1. Technical Field
The present invention relates to a light scanning device for scanning a surface to be scanned by deflecting plural light beams emitted from a light source by a deflection section.
2. Related Art
In an image forming device in an electronographic system, for high resolution and high speed, a light scanning device has been widely used, which simultaneously emits plural light beams from a light source in which plural light emitting elements such as a semiconductor laser is arranged two-dimensionally and deflects them on the same deflection surface to scan a photoreceptor at the same time by plural light beams simultaneously. In addition, as a light source of this light scanning device, a Vertical Cavity Surface Emitting Laser (a so-called VCSEL) has been widely used because of its high degree of freedom in arrangement of the semiconductor laser and low manufacturing cost.
In the light scanning device, generally, a photodetector is arranged so that light beam at starting of a main scanning can enter upon. This photodetector generates a scanning onset signal (hereinafter, called as a SOS signal) in accordance with timing of detection of the scanned beam and a driving circuit of a light source controls a start position of the main scanning based on a SOS signal generated by the photodetector.
As shown in FIG. 9, as a photodetector, one configured by a photodiode applying a current depending on a light incident amount, an amplifier OP for perform I (current)/V (voltage) conversion by amplifying the inputted current, a threshold power source SP for generating a voltage indicating a threshold, and a comparator CP for comparing each output voltage of the amplifier OP and the threshold power source SP has been widely known. In this photodetector, the SOS signal is made a high level when the output voltage of an amplifier OP is not less than a threshold.
It is general that a single mode oscillation (oscillation in a single wave length) is required for the light scanning device in order to obtain a minute beam spot, however, if the single mode oscillation is made in the VCSEL, there is a tendency that the light emission output is small. Therefore, in the case of scanning a photodiode PD by lighting only one VCSEL, the light energy amount received by the photodiode PD is small. So it may be required that the amplification gain of the photodiode PD is increased or the threshold voltage is decreased. However, in this case, this makes the scanning easily affected by the noise. Therefore, a method to increase a light energy amount received by the photodiode PD by lighting plural VCSELs of which positions in the main scanning directions are close to each other and scanning the photodiode PD is devised.
According to this method, when there is no displacement in the positions in the main scanning directions of plural VCSELs (Δ=0 μm), plural light beams emitted from the plural VCSELs at the same time enters the photodiode PD at the same time, so that, as shown in a graph of FIG. 10, a received light energy profile is formed, which has one rising and one falling and has the maximum value larger than the light emission energy amount of each VCSEL. Therefore, there are only two cross points between the received light energy profile and an energy level corresponding to a threshold vale of generation of a SOS signal without raising the amplification gain of the photodiode PD or lowering the threshold voltage, and this makes it possible to generate a SOS signal stably. Further, the graph of FIG. 10 shows a received light energy profile in a main scanning direction in the photodiode PD when three VCSELs having Gaussian distribution with a beam diameter of 60 μmare lighted at the same time to scan the photodiode PD.
However, if there is a displacement in the positions of the main scanning directions of plural VCSELs (Δ=100 μm), there is a difference in times that plural light beams emitted from the plural VCSELs at the same time enter the photodiode PD. Therefore, as shown in the graph of FIG. 10, rising and falling are repeated for each light beam and the received light energy profile of which the maximum value is substantially equivalent to the light emission energy amount of each VCSEL is formed. Therefore, in order to make only two cross points between the received light energy profile and the energy level corresponding to the threshold vale of generation of the SOS signal, the amplification gain of the photodiode PD should be increased or the threshold voltage should be increased, and this makes the affect of the noise easy to receive.